Field of the Invention
The present invention relates to an apparatus and a method for transferring a workpiece such as a semiconductor wafer to and from a storage section and, more particularly, relates to an apparatus and a method for transferring a workpiece to a prescribed position in a storage section.
A process of manufacturing semiconductors includes a step of transferring a sheet of semiconductor wafer from a cassette to a processing chamber or a step of transferring a sheet of semiconductor wafer from one processing chamber to another processing chamber. FIG. 1 is a perspective view of a transfer mechanism of the art typically used for such transferring steps.
The transfer mechanism 1 includes an arm 2 and a semiconductor wafer W can be held at a tip end portion thereof. At this portion of the arm 2, a recess for containing a wafer is provided to hold the wafer therein (see FIG. 3). Another end portion of the arm 2 opposite to the tip end portion is pivotally connected to one end portion of an auxiliary arm 3, and the other end portion thereof is pivotally connected to an auxiliary arm at one end portion thereof. The other end of the auxiliary arm 4 is connected to a rotating mechanism 5 for rotating the auxiliary arm 4. Although not shown, a guide is provided for allowing the arm 2 to be reciprocated only in the axial direction thereof. Consequently, when the auxiliary arm 4 is rotated by the rotating mechanism 5, the arm 2 can be reciprocated between a wafer supplying position and a retracted position of the arm while holding a wafer thereon (or not holding a wafer thereon when the wafer is processed in a processing chamber).
FIG. 2 is a perspective view of the transfer mechanism and shows a configuration thereof when the arm with a wafer held thereon is at the retracted position. The wafer can be transferred from one processing chamber to another processing chamber by rotating or moving the transfer mechanism as a whole while keeping such a configuration (hereinafter such rotation or movement is simply referred to as "movement").
FIGS. 3A, 3B, 3C and 3D and FIGS. 4A, 4B and 4C show operations of unloading and loading a wafer from and to a processing chamber using the transfer mechanism of the art. AS shown in FIG. 3A, a wafer W is supported by a plurality of pins 22 such that the center of the wafer W is aligned with the center of a wafer stage 21 in the processing chamber (not shown). After processing, in order to unload the wafer W therefrom, the arm 4 is rotated as shown in FIG. 1, and the arm 2 is simultaneously extended. The arm 2 extends to a position (an extended position) at which the center of the wafer holding portion of the arm 2 (indicated by a white triangle in the figure) is aligned with the center of the wafer W (indicated by a black triangle in the figure). Then, the pins 22 are moved downward as shown in FIG. 3B to place the wafer W in the recess of the arm 2. The arm 4 is then rotated to return the arm 2 to the retracted position while holding the wafer W thereon, as shown in FIG. 3C.
In order to subsequently process the wafer W, the transfer mechanism as a whole is moved (rotated) as shown in FIG. 2 while the center of the wafer holding portion is aligned with the center of the wafer W held thereon as shown in FIG. 3D, to direct the wafer W to a second processing chamber.
After directed to the second processing chamber, as shown in FIG. 4A, the arm 4 is rotated again to extend the arm 2 to a wafer processing position (at which the center of the wafer stage is aligned with the center of the wafer) in the second processing chamber. Then, the pins are moved upward to transfer the wafer to the wafer stage (FIG. 4B), and the arm 4 is then rotated to return the arm 2 to the retracted position. The processing position and the retracted position are stored in a controller for the transfer mechanism to repeat the unloading and loading operations for other wafers.
As an alternative to such a transfer mechanism as illustrated, there is a transfer mechanism with a vacuum absorption system, and a wafer is held on an arm using vacuum absorption. In this system, a wafer holding portion of the arm is in communication with a vacuum source to hold a wafer by acting the vacuum pumping force to the wafer positioned on the arm. Thus, because a wafer is held by the vacuum pumping force, the need for the recess on the arm to hold a wafer is eliminated so as to allow the arm to be made smaller, and a wafer can be moved or rotated at a high speed.
When a wafer is unloaded by a transfer mechanism of the art (FIG. 1), a wafer on the wafer stage is transferred to the recess of the arm by moving the pins downward so as to position a wafer in the recess of the arm. However, for example, in a processing chamber in which plasma is used for a predetermined process of a wafer, the wafer may be dislocated by the influence of electrical charges and the like caused by the generation of plasma.
The size of the recess formed on the arm 2 is slightly larger than a wafer to be contained therein (normally larger by a gap distance of about 2 mm or less in the radial direction) on the consideration of mechanical errors in the connection between the arms and in the rotating mechanism and the like. However, if the wafer is dislocated beyond the gap distance, the wafer is stranded on the step of the recess so that the wafer can not be contained in the recess, and thereby an error is caused in the wafer transfer. It seems that such a problem can be avoided by predicting the dislocation of the wafer and increasing the size of the recess. However, the wafer can be dislocated inside the recess, and as a result, the reliability of positional accuracy and the repeatability are reduced.
In addition, because the size of the recess is larger than that of a wafer, the wafer can be slid inside the recess during the reciprocation and the rotation of the arm at the retracted position, and thereby the wafer can be dislocated. The speed of movement (rotation) of the arm must be reduced to prevent such a dislocation of the wafer, and thus, it can not be achieved to increase the wafer transfer speed and to shorten the time taken for transferring the wafer. Meanwhile, the arm using a vacuum absorption system does not have such a problem and allows a high transfer speed. However, it is likely to produce particles when a surface of a wafer is strongly contacted with the arm by the vacuum pumping force. Inherently, such a system is only used in the atmosphere.
The invention has been conceived to solve the above-described problems, and it is an object of the invention to provide a mechanism and a method for transferring a workpiece so as to place the workpiece at a prescribed position on an arm member for holding the workpiece without any additional step.
It is another object of the invention to provide a mechanism and a method for transferring a workpiece so as to easily transfer the workpiece to an arm member and place the workpiece at a prescribed position on the arm.
It is still another object of the invention to provide a mechanism and a method for transferring a wafer so as to transfer the wafer from one storage section to another storage section at a high speed.
It is still another object of the invention to provide a mechanism and a method for transferring a workpiece so as to transfer the workpiece under not only a normal pressure condition but also a vacuum condition.
It is still another object of the invention to provide a mechanism and a method for transferring a workpiece which can be adapted for existing apparatuses.